There are nearly a hundred desktop themes ready to access in Cinnamon 18.3 simply go to themes and at the top is add/remove and follow the prompt. Below is the best dark theme I could find there (Eleganse-dark) but the light theme below (Void) is fast becoming my favorite. I am using X2 scaling on a 40" screen. The panel is enlarged by 75% of its maximum height.

I use a slideshow as the desktop. I got most photos free at Wallhaven.com. They have all genre's and thousands of photos in a large format perfect for desktops. There is no registration and never an email from Wallhaven. Create your own slideshow. Easy Peasy.

click on the photos for a better view

The Eleganse-dark menu has some transparency which you may barely see in the photo.

The theme Void has numerous 3-D panels like the menu, panel, time, audio, and wyfi